This invention relates to a laser wavelength conversion apparatus designed to be able to perform wavelength conversion with high efficiency even when the repetition frequency of laser light is varied dynamically.
It has been common practice to perform wavelength conversion of the wavelength of laser light emitted from a laser oscillator, thereby obtaining laser light with a short wavelength. In performing exposure for production of a high density LSI, for example, ultraviolet laser light is required. Thus, it has been done to convert infrared laser with a long wavelength, which has been generated by a solid state laser oscillator, into laser light with a short wavelength by a laser wavelength conversion apparatus, thereby obtaining ultraviolet laser light.
A laser wavelength conversion apparatus has a nonlinear optical crystal device, a wavelength conversion element. When laser light is incident on the entrance end surface of the nonlinear optical crystal device, laser light with a wavelength shortened as a result of wavelength conversion is delivered from the exit end surface of the nonlinear optical crystal device. KTP (KTiOPO4), LBO (LiB3O), BBO (xcex2-BaB2O6), and CLBO (CsLiB6O10) are known as such a nonlinear optical crystal device.
The above-mentioned nonlinear optical crystal device is cut so as to fulfill the conditions for phase matching at a preset temperature. Making the set temperature several tens of degrees higher than room temperature brings the benefit that influence by fluctuations in room temperature can be minimized. Depending on the type of the device, darkening of the crystal device many minimally occur at high temperatures. For example, it has been recommended to set the temperature at 80xc2x0 C. for KTP (KTiOPO4), LBO (LiB3O), and BBO (xcex2-BaB2O6), and at 150xc2x0 C. for CLBO (CsLiB6O10).
A conventional laser wavelength conversion apparatus 1 will be described with reference to FIG. 9, a front view. A nonlinear optical crystal device (a wavelength conversion element) 2 is fixed to a holder 3, and a heater 4 is disposed below the nonlinear optical crystal device 2 and in the interior of a lower portion of the holder 3. A thermocouple 5 is installed between the heater 4 and the nonlinear optical crystal device 2. The outer periphery of the holder 3 and the heater 4 is surrounded with a heat insulator 6.
In FIG. 9, laser light advances along a direction perpendicular to the sheet surface, enters the entrance end surface of the nonlinear optical crystal device 2 (i.e., the face side of the sheet surface of FIG. 9), and passes through the interior of the nonlinear optical crystal device 2. During this passage, the laser light undergoes wavelength conversion, and exits from the exit end surface (the surface located at the farthest position on the back side of the sheet surface).
A heater controller 10 regulates heat generation of the heater 4 by turning on and off an electric current fed to the heater 4, and a detection signal on a temperature detected by the thermocouple 5 is transmitted to the heater controller 10.
With the above-described conventional laser wavelength conversion apparatus 1, the heater 4 generates heat based on control by the heater controller 10 to heat the nonlinear optical crystal device 2 and the holder 3 and raise their temperatures. When their heating and temperature raising are complete, they are thermally insulated and kept warm by the heat insulator 6. Thus, heat dissipation to the outside is suppressed, and the temperature of the nonlinear optical crystal device 2 becomes stable. A solid state relay is incorporated in the heater controller 10, and this solid state relay is on-off controlled so that the value of the temperature detection signal (i.e., temperature) from the thermocouple 5 reaches the set temperature.
Conventionally, only cases in which laser light of a constant repetition frequency is continuously incident have been defined. Thus, the amount of heat input in accordance with the absorption of some of laser light to the nonlinear optical crystal device 2 during laser light entry was constant. Therefore, the temperature of the nonlinear optical crystal device 2 was stabilized merely by on-off control of the solid state relay. That is, the temperature of the nonlinear optical crystal device 2 could be maintained at such a temperature as to maximize the conversion efficiency, or at a temperature in the vicinity of this temperature.
Recently, however, the necessity has arisen for performing an operation for dynamically varying the repetition frequency of incident laser light at the user""s request. When the repetition frequency of laser light is varied dynamically, the amount of heat input associated with the absorption of laser light to the nonlinear optical crystal device 2 increases or decreases in accordance with the variation of the repetition frequency.
In increasing the repetition frequency of laser light, in particular, the amount of heat input to the nonlinear optical crystal device 2 increases, thus arousing the need to cool the nonlinear optical crystal device 2. Simply stopping the heating of the heater 4, however, still posed difficulty in promptly cooling the nonlinear optical crystal device 2 to a temperature range in which the conversion efficiency is satisfactory, because the heat insulator 6 is present around the nonlinear optical crystal device 2. In other words, the outcome was poor temperature stability.
The present invention has been accomplished in view of the foregoing conventional technologies. The object of the invention is to provide a laser wavelength conversion apparatus which can maintain the temperature of a nonlinear optical crystal device stably in a temperature range in which the conversion efficiency is satisfactory, even when the repetition frequency of laser light is varied dynamically.
The present invention is configured to have a wavelength conversion element for performing wavelength conversion of laser light entered from an entrance end surface and delivering laser light of a shortened wavelength from an exit end surface; a heat sink surrounding the peripheral surface of the wavelength conversion element and having cooling fins; a heater for uniform heating disposed in the heat sink in such a state as to surround the periphery of the wavelength conversion element; a temperature sensor for measuring the temperature of the wavelength conversion element; and a heater controller for controlling an electric current supplied to the heater for uniform heating so that the temperature detected by the temperature sensor becomes a preset temperature.
Because of this configuration, when the repetition frequency of laser light is high, heating by the heater for uniform heating is stopped, whereby satisfactory cooling is performed by the heat sink having the cooling fins. As a result, the temperature of the wavelength conversion element can be brought to the set temperature at which the conversion efficiency is satisfactory. When the repetition frequency of laser light is low, on the other hand, heating by the heater for uniform heating is carried out, whereby the temperature of the wavelength conversion element can be brought to the set temperature at which the conversion efficiency is satisfactory. Consequently, always satisfactory wavelength conversion can be achieved, even when the frequency of incident laser light is varied dynamically.
The present invention is also configured such that of the cooling fins of the heat sink, the cooling fins located on side surfaces are arranged in such a state as to extend in a vertical direction.
Thus, air heated by the cooling fins ascend naturally as an ascending air stream, thus increasing the cooling efficiency.
Moreover, the upper and lower fins and the fins on the side surfaces are perpendicular. Thus, there is no inflow of a heat release air stream between these fins, so that cooling by the respective fins takes place effectively.
The present invention is also configured such that the heater for uniform heating is a plurality of rod-shaped heaters arranged in the heat sink at equal intervals in such a state as to surround the periphery of the wavelength conversion element and in such a state as to extend in the direction of an optical axis.
The present invention is also configured such that the heater for uniform heating is a film-shaped heater disposed in such a state as to surround the outer peripheral surface of the heat sink.
Thus, the wavelength conversion element can be heated uniformly, so that satisfactory wavelength conversion can be ensured.
In the present invention, heaters for temperature gradient correction are placed on the entrance side end surface and the exit side end surface of the heat sink, and the heater controller exercises temperature control such that the amount of heat generation from the heater for temperature gradient correction on the entrance end surface side is larger than the amount of heat generation from the heater for temperature gradient correction on the exit end surface side.
Thus, the temperature gradient in the direction of the optical axis can be eliminated, and the efficiency of wavelength conversion can be further improved.
The present invention is also configured such that a loop gas pipe for blowing a cooling gas uniformly from surroundings toward the exit end surface of the wavelength conversion element is disposed on the exit end surface side of the heat sink.
Thus, the temperature gradient in the direction of the optical axis can be eliminated, and the efficiency of wavelength conversion can be further improved.
The present invention is also configured such that the wavelength conversion element is divided along the direction of the optical axis, and an anti-reflection coating or optical polishing is applied to the end surfaces of the resulting divisional wavelength conversion elements.
Thus, the loss can be decreased.